36 ANIMALS OF THE PAST 



soft or moist earth into which wind-driven 

 dust or sand might lodge, or sand or mud be 

 swept by the mimic flood caused by a thunder 

 shower. 



So there are tracks in strata of every age ; 

 at first tliose of invertebrates : after the worm 

 burrows the curious comphcated trails of ani- 

 mals beheved to be akin to the king crab ; 

 broad, ribbed, ribbon-like paths ascribed to 

 trilobites ; then faint scratches of insects, and 

 the shallow, palmed prints of salamanders, and 

 the occasional slender sprawl of a lizard ; then 

 footprints, big and Httle, of the horde of Di- 

 nosaurs and, finally, miles above the Cambri- 

 an, marks of mammals. Sometimes, hke the 

 tracks of salamanders and reptiles in the car- 

 boniferous rocks of Pennsylvania and Kansas, 

 these are all we have to tell of the existence 

 of air-breathing animals. Again, as with the 

 iguanodon, the foot to fit the track may be 

 found in the same layer of rock, but this is not 

 often the case. 



Although footprints in the rocks must often 

 have been seen, they seem to have attracted lit- 

 tle or no notice from scientific men until about 



